DAC controlled power supply.

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Hello,i have a schematic of a power supply,its output is controlled from a voltage 0-5V and outputs 0-30V.
Is offset trimmed but as output exceeds 7v the offset is getting higher and higher and reaches 100mV or so.
Below 7V has very good accuracy.In any simulation works fine,but in practice has quite big differences in voltage output up to 500mV.
In lack of LT1636 as seen in schematic i use OP07 and Q2 is changed to BD244.
My question is,if is possible for the output to follow exactly the reference voltage by a ratio of 6 in the entire range from 0 to 30V.

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I cannot tell what your reference voltage is because it is not labeled. Is it V3?

I also suspect that a real circuit will have some big stability problems. Too many poles.

It might work over the 0-30V range, but there will be a BIG problem with power dissipation. A 10 ohm resistor means 1.5A of current @ 15V out. The 2N3055 will have a voltage drop of 25V. That makes 37.5W dissipated in that transistor. But, then, simulation only has virtual smoke; you won't smell real smoke until you build it.

Jim

 

Until Black Lives Matter, we do not have "All Lives Matter"!

 

 

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Though by ALT-clicking on a component you add a power-dissipation graph. With a bit of common sense you can figure out if the reported dissipation means smoke. When simulating LT's switcher ICs an exclamation mark will appear next to the component if it gets too hot or when other absolute maximum ratings are exceeded.

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Thank you for your response.V3 is the reference voltage,in practice i dont connect 7ohm load but 1.5k
and losses are negligible,also i dont use V5,R12,M3,R11,these are in simulation only for adding a ripple in the output.But the problem is that output in higher than 7 V is not exactly proportional to reference voltage as i was expected.I am chasing some millivolts that are missing.In the worst case is 0.5V.

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Maybe R1, R2 are heating and drifting with increased current flowing? I would use low tempco resistors for those two resistors just to see what happens. It is not clear how the reference works. So it is hard to tell if changes on the output would effect the reference voltage. Certainly the opamp input would not bother it. You might put a DVM on that reference input to the LT1636 and confirm it is rock solid.

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The reference in practice is the unbuffered output of a summing amplifier.Tomorrow i will replace it with a multiturn potensiometer to see the results.A buffering stage maybe needs?

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What is the input of the summing junction coming from? What about the power supply to the summing junction? Are these nodes stable as the power supply is ramped up to full scale?

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The opamp You have selected has a gain bandwidth product of 100KHz.

Low gain bandwidth performance may reflect in less than tight gain performance.

Why is R3 set to 220K?

The opamp can deliver up to 18mA. Reduce the gate drive resistor to something more civilised.

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What is the idea behind the three transistor output stage? Seems like you could easily get away with eliminating Q2...

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Does the error occur at light loads?

I recall there being something very Not Good about 2N3055s. Maybe a drastic drop in beta at high currents? I recall it being quite nonlinear. More modern NPNs might be a much better choice (3055's are circa 1960s, I believe).

Lets check some numbers. Suppose that the beta at 1A is 50 (it might be even less_. That means a base current of 1A/50 = 20ma. The 47 ohm R5 takes about 0.7V/47 ohms = 15ma or so. So, Q2 would have to deliver 35ma, more or less. At these currents, the beta of a 2N2907 should be more than 100. That means a base current around 350uA, more or less.

A 2N2907 is really optimized for MUCH higher currents (100ma, or more). But, that should not be the cause of problems since it is worst at lower output voltages (where it seems to be more accurate. The absolute maximum base current in Q2 is about 40V/4.7K or about 5mA.

It might be useful to report a full set of node voltages at some point where the output is not what you expect.

Jim

 

Until Black Lives Matter, we do not have "All Lives Matter"!

 

 

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from memory 2n3055 beta less than 20 at elevated collector current

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Sometimes i look but i cant see.I connected wrong a capacitor,before R8 closer to the reference voltage,and this mainly caused the error.
Except this,it might have errors in ground loops and other things also.I started it for fun in the previous August making an oldie goldie power supply based in LM723 with 2N3055.Added Mega8 for voltage and current readings and
later software PWM DAC for controlling voltage and current,eliminatining LM723 but keeping the existing output stage.Q2 as seen in schematic could be eliminated also and Q3 replaced with a PNP power darlington or more.Nevertheless the circuit works as far and could be made better.
Thank you all for your replies and your suggestions.

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